I would think it probably greatly reduced the amount of traffic in that area, which all around just makes for a more pleasant experience being a pedestrian, biker, or scooterer.
Regardless, I think this is awesome and wish it could be tried in the United States. Kids being able to be independent and active is essential to their happiness and development.
https://roadtraffic.dft.gov.uk/regions/6 has a good chart - 20 billion vehicle miles to 19 billion. Interestingly from the chart, local traffic stayed about the same whilst main roads seem to have lost a little.
The ULEZ zone is now basically all of the city, it doesn't quite go to the M25 (motorway ring road) but anywhere that a tourist would even remotely think of as being London is well inside it.
Add to that growing up with a dog or cat (implies parents are well off enough to take care of said animal when the kid is a kid and spaces being responsible) and living when they can play in wild spaces (not manicured lawns), plant flowers, veg, etc. learn getting stung suks but usually not fatal. A big plus is being around livestock and as the kids mature, having an opportunity to take care of said livestock (4H program)
Plenty of wild nature preserves, coyotes running around, rabbits playing along the side walks, and parks all over the place.
It is possible for cities to be built around the concept of being integrated with nature, though in the specific case of London, it is too late to tear down a bunch of houses just to increase the density of parks.
Anecdotally, traffic is worse than ever, school drop off and pickup times are particularly bad. I have on occasion been stuck in school run traffic for literally hours going <5km around Putney / Clapham.
That is why everyone I know buys a 4WD and spends an hour driving their children 500m to and from school every day.